Production of aqueous calcium nitrite solutions by absorbing nitrogen oxide gases into a basic calcium hydroxide solution has been known for some time. Such solutions can be employed as additives in cement formulations. Numerous references abstracted in Chemical Abstracts describe the above technology. Specifically, in volume 77, (1972) an article from Ref. Zh., Khim. 1971, describes the use of milk of lime to trap waste gases from production of nitric acid. The abstract in CA77:38609h is similar. In CA85:145344e nitrogen oxide gases are admixed with a slurry of calcium hydroxide to form aqueous solutions of calcium nitrite. A similar process is described in CA85:145345f. In CA84:61959 (Pure Calcium Nitrite), nitrogen oxide gases are absorbed into a calcium hydroxide slurry which is filtered to give a calcium nitrite solution. The abstract of a journal article appearing in Zh. Neorg. Khim. 1973, 18(12), 3340-1 (Russ) indicates that anhydrous calcium nitrite is prepared by the reaction of calcium hydroxide with ammonium nitrite followed by heating the crystallohydrates in a vacuum.
The calcium hydroxide/calcium nitrite double salt employed in the invention is also well known. See for example, Erhitzen im Luftstrom, F. W. Klingstedt (Acta Acad. Aboenisis 9 Nr.1[1936]1/29). See also L. FERSEN (Zement 24[1935]77/82), and German Pat. No. 649,674 (1936/37) to I. G. Farben.
The U.S. patent literature also discloses various processes wherein nitrogen oxide gases are absorbed into various basic solutions. U.S. Pat. No. 1,471,711 describes absorption into soda and caustic soda solutions to produce aqueous nitrite solutions. U.S. Pat. No. 2,797,144 describes a process for producing ammonium nitrite wherein nitrogen oxide gases with an NO/NO.sub.2 molar ratio of at least 4 are contacted with oxygen and ammonium carbonate or ammonium sulfite. In U.S. Pat. No. 3,928,543 nitrogen oxide gases are contacted with aqueous hydroxide solutions (e.g., calcium hydroxide), and the progressive dilution of the nitrogen oxide content of the gas is correlated with the base content of the solution. In U.S. Pat. No. 4,009,246 high temperature gases containing nitrogen oxide are contacted with aqueous solutions of a base (e.g., calcium hydroxide) to rapidly lower the temperature of the gas and produce nitrite salts containing the cation of the base. U.S. Pat. No. 4,045,540 describes production of ammonium nitrite by absorbing gases containing nitrogen oxides into a basic solution containing ammonium cation. Air pollution from residual nitrogen oxide gases is reduced by adding controlled supplemental quantities of NO.sub.2 to the absorption system. U.S. Pat. No. 4,053,555 describes a system for scrubbing industrial waste gases containing nitrogen oxides with an aqueous solution to absorb the gases. Subsequently, "desorption" is accomplished to regenerate the initial scrubbing solution, which can be recirculated.
In U.S. Pat. No. 3,965,247 sodium nitrite is recovered from aqueous sodium chloride solutions by crystallization.